Recently LED (light emitting diode) light sources have been widely used. Since a large portion of lights of a LED light source is emitted toward the front, an optical element configured to diverge lights from the LED light source is commonly used in a combination with the LED light source. Particularly, when LED light sources are used as light sources of an illumination unit for illuminating a large area, such as that for backlight, optical elements configured to diverge lights from the LED light sources over a large angle are used such that a compact illumination unit can be realized with a small number of LED light sources (for example, Patent Document 1).
Backlight illumination units are divided into two types. One is an edge-light type in which light sources are arranged on a side of a diffusion plate or the like and lights for illumination are sent toward the front by the principal of a light-guiding plate. The other is a direct type in which plural light sources are arranged immediately below a plane to be illuminated. The direct type includes an illumination unit of line arrangement type in which light sources are arranged in a single line or in plural lines.
When LED light sources or the like are used for light sources of such a backlight illumination unit of line arrangement type, the light sources in a line or in lines should preferably be configured to illuminate a large area with uniform light intensity as a whole.
However, when LED light sources or the like are used for light sources of a backlight illumination unit of line arrangement type, optical elements which are specially intended for use with the light sources such as LEDs arranged in a line or in lines for illumination of a large area with uniform light intensity as a whole have not been developed.